Superior performance of novel chitosan-based flocculants in decolorization of anionic dyes: Responses of flocculation performance to flocculant molecular structures and hydrophobicity and flocculation mechanism.

To achieve economical and efficient decolorization, two novel flocculants, weakly hydrophobic comb-like chitosan-graft-poly (N, N-Dimethylacrylamide) (CSPD) and strongly hydrophobic chain-like chitosan-graft-L-Cyclohexylglycine (CSLC) were synthesized in this study. To assess the effectiveness and application of CSPD and CSLC, the impacts of factors, including flocculant dosages, initial pH, initial dye concentrations, co-existing inorganic ions and turbidities, on the decolorization performance were explored. The results suggested that the optimum decolorizing efficiencies of the five anionic dyes ranged from 83.17% to 99.40%. Moreover, for accurately controlling flocculation performance, the responses to flocculant molecular structures and hydrophobicity in flocculation using CSPD and CSLC were studied. The Comb-like structure gives CSPD a wider dosage range for effective decolorization and better efficiencies with large molecule dyes under weak alkaline conditions. The strong hydrophobicity makes CSLC more effective in decolorization and more suitable for removing small molecule dyes under weak alkaline conditions. Meanwhile, the responses of removal efficiency and floc size to flocculant hydrophobicity are more sensitive. Mechanism studies revealed that charge neutralization, hydrogen bonding and hydrophobic association worked together in the decolorization of CSPD and CSLC. This study has provided meaningful guidance for developing flocculants in the treatment of diverse printing and dyeing wastewater.